1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, to a photonic crystal light emitting device with high optical efficiency using photon-recycling.
2. Description of the Related Art
Recently, there is performed a lot of researches on applying a photonic crystal structure as technologies to improve light extraction efficiency of semiconductor light emitting device such as GaN-based light emitting diodes (LEDs). LEDs having a photonic crystal structure may have merits of not only improving light extraction efficiency but also improving internal quantum efficiency and controlling a light emitting direction.
However, due to a dry etching generally used to form a photonic crystal structure, a serious damage occurs in a semiconductor crystal structure for electrical operation. Also, efficient current supply via the photonic crystal structure is another object to be solved by a photonic crystal LED. Now, most of researches on photonic crystal LEDs employ a method of forming a photonic crystal structure, a periodical refractive index modulation structure on only a surface of an LED chip. The photonic crystal LED has an additional structure for efficient current injection. The photonic crystal LED having such a structure is incapable of utilizing intensification of internal quantum efficiency that is one of greatest characteristics of a photonic crystal. Accordingly, it is difficult to provide improvement over conventional corrugated interface surface (CIS) or surface roughness structures.
FIG. 1 is a perspective view schematically illustrating a conventional photonic crystal LED 10. Referring to FIG. 1, the photonic crystal LED 10 includes an n-type GaN layer 13, an active layer 15, and a p-type GaN layer 17, sequentially formed on a sapphire substrate 11. On the n-type GaN layer 13, the active layer 15, and the p-type GaN layer 17, there are arranged holes H with a smaller size than that of a light wavelength in a two-dimensional periodical structure with an interval A to a degree of the light wavelength, to form a photonic crystal structure. The photonic crystal LED 10 has functions of not only improving light extraction efficiency but also improving a light yield and controlling a light emission direction. Also, it has been considered that problems such as a reduction of an active layer area due to the holes H of a periodical arrangement and a surface recombination are eased by an effect of improving internal efficiency due to a photonic crystal structure or a passivation process.
However, this is a conclusion without fully considering that an LED is electrically driven. For example, to improve the light yield, the holes H should be formed to a depth of an area including the active layer 15. Due to a dry etching generally used to form the holes H having the depth, particularly, inductive coupling plasma reactive ion etching (ICPRIE), a serious damage occurs in a semiconductor crystal structure for electrical operations, particularly, a crystal structure around an active layer. Particularly, an n-type donor occurs in a p-doped portion and reduces a doping concentration of the p-type semiconductor layer 17. This phenomenon not only partially occurs but also is longitudinally and laterally spread. Due to this, a semiconductor LED may lose functions of an electrically driven device. Accordingly, most of current researches on photonic crystal light emitting devices employ a method of forming a photonic crystal structure on a surface of a chip, such as a shallow photonic crystal.
FIGS. 2A to 2C are partial cross-sectional views illustrating various conventional photonic crystal structures. As shown in FIG. 2A, the shallow photonic crystal where a depth of an element of a photonic crystal, such as a hole and a post, is below an active layer or a multi-quantum well (MQW) may be used to improve light extraction efficiency. A photonic crystal where a depth or height of an element thereof reaches an active layer or a light emitting layer as shown in FIG. 2B or a strong photonic crystal where an element thereof perfectly penetrates an active layer or a light emitting layer as shown in FIG. 2C may be used. However, the shallow photonic crystal of FIG. 2A does not fully use an effect of strengthening a light yield and the photonic crystals of FIGS. 2B and 2C cause great damages in a semiconductor crystal required for electric drive (refer to FIG. 1).
Another problem of a photonic crystal is present in efficiently injecting a current. Since forming a photonic crystal structure accompanies a partial removal of a current injection path, it is considerably difficult to embody relatively uniform current distribution. Nonuniformity of current distribution has a bad effect on current injection efficiency and decreases internal efficiency of an entire LED. To solve the problem, a transparent metal electrode conformally covering an almost entire of the photonic crystal structure may be used.
FIG. 3 is a cross-sectional view illustrating another conventional photonic crystal LED 20. Referring to FIG. 3, an n-type semiconductor layer 23, an active layer 25, and a p-type semiconductor layer 27 are sequentially laminated on a sapphire substrate 21, and an n-side electrode 29 is formed on the n-type semiconductor layer 23. A shallow photonic crystal structure is formed on a surface of the p-type semiconductor layer 27. To improve lateral current spreading, a thin transparent metal electrode layer 24 conformally covers almost the photonic crystal structure. Just below a p-bonding electrode 28 in contact with the transparent metal electrode layer 24, an insulation body 22 is disposed between a photonic crystal and the transparent metal electrode layer 24 to restrain a current concentration near the p-bonding electrode 28. However, since the photonic crystal LED 20 has the shallow photonic crystal structure, as described above, the effect of increasing a light yield is not fully used. Accordingly, there is obtained little effect over conventional corrugated interface surface (CIS) or surface roughness structures.